The invention relates to an active matrix liquid crystal display, and in particular to an active matrix liquid crystal display incorporating driver circuits for the row and column electrodes.
Active matrix liquid crystal displays are well known. In such displays, an active plate and a passage plate sandwich a liquid crystal. The active plate includes a number of electrodes for applying electric fields to the liquid crystal; the electrodes are generally arranged in an array. Row and column electrodes extending along the rows and columns of pixel electrodes connect and drive thin film transistors which drive respective pixel electrodes.
The row and column electrodes are driven to control the thin film transistors to control the charge stored on corresponding pixel electrodes. Each pixel may also include a capacitor for maintaining charge on the pixel.
One difficulty is in providing the necessary circuits for decoding incoming signals and driving the row and column electrodes. Generally, such drive circuits are arranged around the outside the pixel array.
Some designs of active matrix display include one or more circuit elements in addition to the thin film transistor associated with each individual pixel within the array. For example, U.S. Pat. No. 5,926,158 has a pixel capacitor, memory cell, and a switch associated with each pixel. These circuit elements are formed at the level of the thin film transistor.
However, although providing such additional circuitry associated with each pixel alters somewhat the requirement for driver circuitry, there remains the need for row and column drivers and the need to provide space for them.
There is a need for an improved arrangement to reduce the area taken up by the row and column drivers of an active matrix display.
According to the invention there is provided an active plate for an active matrix liquid crystal display, including a substrate; an array of pixels, each pixel having a pixel electrode for controlling liquid crystal; a plurality of row electrodes and column electrodes extending across the substrate in substantially orthogonal row and column directions connected to the pixels; a sub-array of driver cells extending across the substrate between the array of pixels and the substrate, the driver cells each including at least one driver circuit for driving a row electrode or a column electrode; and an insulating layer between the sub-array of driver cells and the array of pixels; wherein each of the driver circuits in the sub-array is connected to a corresponding one of the row and column electrodes through a respective via defined in the insulating layer extending from the driver circuit to the corresponding one of the row and column electrodes.
Thus, in the invention the driver circuits are arranged in an array under the pixel array, and connected to the row or column electrodes of the pixel array by vias. In this way, the amount of space required for the row and driver circuits around the edge of the array may be substantially reduced from that required in conventional arrangements.
Further, the arrangement according to the invention greatly eases connection between the driver circuits and the row and column electrodes.
It will be noted that since the number of driver circuits required equals the number of row electrodes, or column electrodes, or both, this is much less than the number of pixel elements which is the product of the number of rows and the number of columns. Accordingly, the driver circuits can be significantly larger in size than the individual pixel elements, so that the circuits can readily be implemented using available lithography resolution.
Preferably, the vias are distributed over substantially the whole of the area of the array, rather than being arranged in a particular location, for example around the outside of the array. In this way, in particularly preferred embodiments, the row or column electrodes are connected directly to the driver circuits through the vias without the use of any lateral interconnections in the sub-array of driver cells.
Embodiments of the invention may provide further processing elements on the same level of the sub-array of driver cells. In particular, such further elements may include a memory, for example for use as a frame store to allow the display to run in low power mode when not displaying high quality images.
It will be appreciated that in embodiments of the invention the sub-array can include all the row drivers, all the column drivers, or both. Row or column drivers not provided under the pixel array may be provided around the edge of the pixel array. In this way, the area of the substrate required for driver and driver circuits can be reduced from arrangements requiring both row and column drivers to be arranged around the outside of the pixel array.
An edge sealant may be provided to seal the cell, rather than the more normal sealant that extends into the bulk of the glass by about 1.5 mm.
The invention also relates to a liquid crystal display including an active plate as set out above. The invention allows the pixel elements to extend right to the edge of the substrate of which the active plate of the display is provided.
The invention also relates to a reflective liquid crystal display comprising an array of active plates as described above arranged in a plane, the substrates of the active plates extending in the plane of the array; at least one passive plate spaced transversely away from the two dimensional array; and liquid crystal between the active and passive plates.
The invention also relates to a method of making an active plate for a liquid crystal display including: defining a sub-array of driver cells across a substrate, the driver cells each including at least one driver for driving a row and/or a column electrode; depositing an insulating layer on the sub-array of driver circuits; defining a pattern of vias passing through the insulating layer, the vias being distributed over the sub-array of driver circuits; defining an array of pixels over the insulating layer, the array of pixels including a plurality of row electrodes and column electrodes extending across the substrate in substantially orthogonal row and column directions connected to the pixels; wherein each of the driver circuits in the sub-array is connected to a corresponding one of the row and column electrodes through the respective via defined in the insulating layer.